Various studies were undertaken to investigate the growth potential in athymic mutant nude mice of selected murine tumors and certain human neoplastic cell lines. A lymphosarcoma spontaneously arising in a nude mouse was characterized histologically and ultrastructurally. The cells were grown in tissue culture, and after 6 months typical murine-type viral C- particles appeared. The cells have been maintained both in culture and as transplantable solid tumors. Two human lymphocytoid cell lines, a reticulum cell sarcoma and a Hodgkin's disease line were established in culture. Both lines were inoculated into nude mice and maintained as transplantable solid tumors. A human cell line initially derived from an undifferentiated lung carcinoma that produced large quantities of human chorionic gonadotrophin was inoculated into athymic mice. The solid tumors resulting from the inoculation were carried for several generations. Tumor-bearing animals developed high blood levels of chorionic gonadotrophin. Primary tissue cultures of nude mouse kidney were established and infected with a human papovavirus isolated from a patient with Wiscott- Aldrich syndrome and multiple malignancy. The cells underwent malignant transformation, and established slow-growing transplantable solid tumors upon inoculation into nude mice.